m-Phenylenediamine has been commonly produced by the reduction of m-dinitrobenzene. This process is relatively expensive in that the m-dinitrobenzene utilized is obtained through the complete dinitration of benzene which is then separated and purified prior to the reduction. This purification involves reaction with a substance such as sodium sulfite which places the undesirable o-dinitrobenzene and p-dinitrobenzene into an aqueous phase resulting in a large amount of waste water as disclosed, for example, in U.S. Pat. No. 3,086,063. The m-phenylenediamine must then be separated from reaction impurities after the reduction. m-Phenylenediamine has also been produced by reduction of dinitrobenzene, followed by separation of m-phenylenediamine from o-phenylenediamine and p-phenylenediamine. Although less expensive than the above method, the dinitrobenzene utilized is obtained through complete dinitration of benzene which must be extracted or separated as a solid from the reaction medium.
More recently, U.S. Pat. No. 4,185,036 disclosed the hydrogenation of a mixture of an aromatic mononitro compound and an aromatic dinitro compound in the liquid phase. According to the process, hydrogen and a homogeneous or heterogeneous liquid mixture of at least 25 wt. % of an aromatic dinitro compound and/or at least 25 wt. % of a mononitro monoamino compound and at least 25 wt. % of an aromatic mononitro non-amino compound are reacted with vigorous mixing in the presence of a hydrogenation catalyst at a temperature of from about 75.degree. to 225.degree. C. and a pressure of about 50 to 800 psi. According to the patent, pure or relatively pure nitro products are used in the process as the starting materials to produce a mixture of the monoamino and diamino compounds which are then separated by distillation. Accordingly, the process is relatively expensive because of the need to separate and purify the starting materials and then the final products; and, additionally, the reaction rates in the liquid-phase hydrogenation are relatively slow, resulting in high energy consumption and low reactor utility.